a review on exergy analysis of solar refrigeration...

Industrial Engineering 2020; 4(2): 14-32

http://www.sciencepublishinggroup.com/j/ie

doi: 10.11648/j.ie.20200402.11

ISSN: 2640-110X (Print); ISSN: 2640-1118 (Online)

A Review on Exergy Analysis of Solar Refrigeration Technologies

Paiguy Armand Ngouateu Wouagfack1, *

, Maurice Tenkeng2, 3

, Daniel Lissouck1, Réné Tchinda

2, 3

1Department of Renewable Energy, Higher Technical Teachers, Training College, University of Buea, Kumba, Cameroon 2L2MSP, Department of Physics, University of Dschang, Dschang, Cameroon 3LISIE, University Institute of Technology Fotso Victor, University of Dschang, Dschang, Cameroon

Email address:

*Corresponding author

To cite this article: Paiguy Armand Ngouateu Wouagfack, Maurice Tenkeng, Daniel Lissouck, Réné Tchinda. A Review on Exergy Analysis of Solar

Refrigeration Technologies. Industrial Engineering. Vol. 4, No. 2, 2020, pp. 14-32. doi: 10.11648/j.ie.20200402.11

Received: March 11, 2020; Accepted: April 24, 2020; Published: August 27, 2020

Abstract: Solar energy is becoming more and more useful in the modern day life in industrial, domestic and commercial sectors, because of his cleanliness from an environmental point of view and also contributes to the reduction of greenhouse effect

gases such as CO2. Exergy analysis is a thermodynamic analysis technique based on the Second Law of Thermodynamics, which

provides an alternative and illuminating means of assessing and comparing processes and systems rationally and meaningfully.

Exergy analysis can assist in improving and optimizing designs. In this paper, the exergy analysis of solar thermal refrigeration

cyles is reviewed. A review of the research state of art of the solar absorption and adsorption refrigeration technologies is also

carried out. The cycles involved in these technologies are: open, closed, continuous and intermittent cycles. An overview of

mesures of merit with regard to exergy (exergetic efficiency, exergy losses, exergy improvement and exergetic coefficient of

performance) is presented. Besides, an historical and chronological view is done on the development scenario of exergy analysis

in the world from 1824 until 2014. The main mathematical relations for the simulation of those cycles are presented.

Keywords: Exergy Analysis, Solar Refrigeration, Absorption, Adsorption

1. Introduction

Solar energy is becoming more and more solicited by

industries in general and for houses purposes in particular. The

fight for a clean environment and activities without warming is

nowadays a necessity. That’s why refrigeration systems is a

great preoccupation for engineers in laboratories. Recently, the

use of solar energy for both heating and cooling applications

has received more attentions, this is because, the electricity

demands for both heating and cooling by the domestic sector,

during winter and summer, respectively, are quite high. The

conventional electric heater and air conditioner used in the

buildings/domestic sector consume a lot of electricity [1]. In

the countries producing oil in the world, most of their

electricity demands are supplied by conventional power plants

driven by fossil fuels. The problem is that oil will be ceased to

dominate as the main energy source by the end of the 21st

century. Also, with the growth of world’s population and civil

modernization, the world energy demand will definitely

escalate in the next 20 years [2]. With the decrease of fossil

fuel resources, the energy demand is rapidly increasing, and

this will definitely lead to an energy crisis in the near future [3].

Therefore, the coupling of the renewable energy sources such

as solar energy with either heating or cooling systems is a

promising alternative method. Because of the desirable

environmental and safety aspects, it is widely accepted that

solar energy should be utilized instead of other alternative

energy forms as it can be provided sustainably without

harming the environment [4]. Furthermore, fossil fuel

combustion can cause greenhouse effect that highly

contributes to the global warming. There are plenty of

technologies currently available to capture and hold the sun's

power for uses such as water heating, cooking, space heating,

power generation, food drying and refrigeration [5].

The concept of exergy was put forward by Gibbs in 1878. It

was further developed by Rant in 1956, who used the term

‘exergy’ for the first time which refers to the Greek words ex

15 Paiguy Armand Ngouateu Wouagfack et al.: A Review on Exergy Analysis of

Solar Refrigeration Technologies

(external) and ergos (work). The term exergy relates to ideal

work and exergy losses relate to lost work [6]. Szargut [7]

defined the exergy of a system at a certain thermodynamic state

as the maximum amount of work that can be obtained when the

system moves from that particular state to a state of equilibrium

with the surroundings. As it is well known, exergy is the

measure of useful work or potential of a stream to cause change.

Besides it is an effective measure of the potential of a substance

to impact the environment [8, 9]. M. Tenkeng et al. [10]

conducted a study on exergy analysis of a single effect solar

absorption refrigeration system in Ngaoundere, Cameroun. The

exergy loss of each component of the system was analysed in a

half hourly basis. They observed that the evaporator and

generator registered the great losses and were the components

that needed to be optimized. Tenkeng et al. [11] also presented a

study on exergy analysis of a series double effect solar

refrigeration in Ngaoundere, Cameroun on a half hourly basis,

from 6.30AM to 6.30PM. A thermodynamic analysis was

undertaken by simulation with a FORTRAN program and the

results showed that the High Pressure Generator and the

evaporator were the component needing a great attention in term

of exergy efficiency and exergy loss.

The aim of this paper is to provide basic background and

review existing literatures on exergy analysis of solar

refrigeration technologies. A number of refrigeration systems

is also presented and it is hoped that, this should be useful for

any newcomer in the field of solar refrigeration technology

and of great help for investigators on the field of exergy

analysis in the future.

2. Presentation of Solar Refrigeration

Technologies

Solar refrigeration has become more attractive for cooling

purposes today because of the use of desiccant gases, such as

LiCl (lithium chloride) and LiBr (lithium bromide), or water

instead of harmful Freon gas. Cooling can be achieved through

two basic methods. The first is a PV (Photovoltaic)-based solar

energy system, where solar energy is converted into electrical

energy and used for refrigeration much like conventional

methods [12]. The second one utilizes a solar thermal

refrigeration system, where a solar collector directly heats the

refrigerant through collector tubes instead of using solar electric

power [13]. According to Hassan et al. [14] there are three types

of cooling production devices: closed cycle continuous

absorption cooling system, closed cycle intermittent absorption

cooling system and open cycle absorption cooling system. The

intermittent cycle may work based on the single or the double

stage technology. In an intermittent absorption system, there is

no moving part because the solution pump is eliminated and the

density difference is utilized for the solution circulation based on

the thermosiphon principle. The closed cycle continuous

absorption refrigeration systems require heat source

temperatures that are significantly higher than the temperatures

of corresponding condenser. Our review is focusing on the

thermal method and this presentation is focusing on both closed

and open cycles. Absorption refrigeration technologies consist

of a generator, a pump and an absorber that are collectively

capable of compressing the refrigerant vapour. The evaporator

draws the vapour refrigerant [15]. According to the solution

regeneration and thermal operation cycle, the absorption systems

can be divided into three categories: single-effect, half-effect and

double-effect solar absorption cycles. The single-effect and half-

effect chillers require lower temperatures with respect to a

double effect-chiller [16]. There are also two other absorption

refrigeration systems (DAR (diffusion absorption refrigeration)

and hybrid systems) that can achieve better performance [17].

Therefore, the finding of root cause of irreversibilities can be

helpful in the optimum designing and effective utilization of

energy in various applications. Moreover, the exergy analysis

also provides true sense of diversion of existing system from the

ideal one [18]. The solar energy can be utilized (directly or

indirectly) in different applications such as solar drying, solar

refrigeration and air conditioning, solar water heating, solar

cooking and solar power generation.

The very important advantage of the solar absorption

cooling technology is that their coefficient of performance is

higher than that of other thermally operated cycles.

Furthermore, the freedom from noise and vibrations, long

lasting, cheap maintenance, and most importantly the

possibility of using any type of heat source, including solar

radiation and geothermal or waste heat, to energize the

system and provide reliable cooling [14].

Absorption is the phenomenon in which a substance in one

state interpenetrates and incorporates into another of a different

state. The two phases present a strong affinity to form a solution

or a mixture. This process can be reversed and the absorbed

phase can be released from the absorbent by applying heat to the

mixture. The first absorption cooling machine using water as the

absorbent and ammonia as refrigerant was patented in France in

1859 by the French scientist Ferdinand Carre´. The solar

absorption cooling system uses two working fluids, the

refrigerant and the sorbent, in a closed or open mode cycle.

Absorption machines are thermally activated, and for this reason,

high input shaft power is not required [19]. Its working principle

is the same as the vapour compression machine except the fact

that the mechanical compressor replaced the thermal compressor

[5]. The applications of these cooling systems are wide and

include freezing, cooling, and airconditioning. This type of open

absorption cooling system is open to the regenerator side. A

schematic diagram of the solar-operated open absorption

refrigeration system is shown in Figure 8. In this type of cooling

machines, the dilute absorbent solution is heated and

subsequently re-concentrated in the solar collector, which is

open to the atmosphere, due to the evaporating process [5]. In

the open cycle absorption cooling system, one can have two

possible options depending on whether the absorption system is

made open from the regenerator part or from the evaporator part.

The first option is the open regeneration absorption cooling

system, where the system is made open to the regenerator side.

The second option is the open evaporative cooling system, in

which the system is open to the evaporator part [20]. There are

three types of cooling production devices: closed cycle

Industrial Engineering 2020; 4(2): 14-32 16

continuous absorption cooling system, closed cycle intermittent

absorption cooling system and open cycle absorption cooling

system [5]. A comparative study of the open and closed cycle

options has been made. It is concluded that the hybrid double-

absorption solar cooling systems are better in performance than

conventional systems and an open-cycle double-absorption

system is even more attractive and cost effective as compared to

closed-cycle option. [21].

2.1. Solar Absorption Refrigeration Systems

2.1.1. Continuous Operation Systems

i. Half Effect Absorption Refrigeration Systems

The primary feature of the half-effect absorption cycle is

the running capability at lower temperatures compared to

others. The name “half-effect” arises from the COP, which is

almost half that of the single-effect cycle [5].

R. Maryami et al. [22] proposed the schematic diagram of

Figure 1 showing the half effect absorption refrigeration cycle

which consists of two solution circuits, one of them operates

between medium and high pressure level and the other one

operates from low to medium pressure level. Therefore, main

components of half effect cycle are two generators, two

absorbers, two solution heat exchangers, and only one condenser

and one evaporator. In this cycle, high generator (HG) and

condenser operate at high pressure side while low generator (LG)

and high absorber (HA) operate at medium pressure level. On

the other hand, low absorber (LA) and evaporator operate at low

pressure side. In this cycle, the heat transferred to both generator

can be same temperature. An amount heat supplied to HG to

separate refrigerant vapour and once refrigerant vapour is

produced (13), it is condensed in condenser (14) and by passing

from refrigerant expansion valve (15) flows into evaporator. The

evaporation process in evaporator leads to liquid-vapour mixture

converts to refrigerant vapour which it after leaving the

evaporator goes to LA (16). At LA the refrigerant vapour is

absorbed by the solution with low concentration coming from

LG (6). Then the solution produced with intermediate

concentration (1) is pumped to the LG, passing through a

solution heat exchanger I (1-2-3). By means of the heat supplied

to LG, part of the refrigerant separates from solution (17) and

flows directly to HA where it is absorbed by the solution coming

from HG (12). Finally, the LiBr/water solution formed with high

concentration (7) is pumped to HG through the solution heat

exchanger II starting the cycle again (7-8-9). In order to reduce

the heat consumption in the generators solution heat exchangers

are used between the absorbers and the generators to recover

heat from the solution going from the generators to the absorbers.

ii. Single Effect Absorption Refrigeration Systems

Romero et al. [23] presented a single-effect absorption

cooling system as a device which is simpler than others when

the design depends on the types of working fluids. The

system shows better performance with non-volatile

absorbents such as LiBr/water. If a volatile working pair such

as ammonia/water is used, then an extra rectifier should be

used before the condenser to provide pure refrigerant.

Boopathi et al. [24] proposed the schematic of a single effect

lithium bromide absorption cycles of Figure 2.

iii. Double Effect Absorption Refrigeration Systems

a) Series Double Effect Absorption Refrigeration Systems

R. Maryami et al. [22] proposed the line diagram of a series

double effect absorption refrigeration cycle, employing two

generators, two solution heat exchangers, two solution

reducing valves, two refrigerant expansion valves, two

condensers, an evaporator, an absorber and a solution pump as

the main components. The cycle implies three pressure levels,

the high condenser (HC) and HG operate at the high pressure

part of the equipment and low condenser (LC) and LG at the

medium pressure side while the evaporator and the absorber

operate at the lower pressure side. As shown in Figure 3 the

vapour refrigerant coming from evaporator (10) is combined

with strong solution (6) in absorber and the produced weak

solution (1) is pumped from absorber to HG through heat

exchangers I and II (1-2-3-13). In HG a part of refrigerant

vapour boiled out of the solution at relatively high temperature.

The primary vapour (17) and the medium concentration

solution (14) from HG come into the HC and LG, respectively.

The medium concentration solution goes into LG through the

heat exchanger II and solution reducing valve (14-15-16). Heat

rejected from primary vapour in HC is transferred to the

medium concentration solution in LG and this heat transfer

leads to vapour being g condensed. The secondary vapour

produced at outlet of LG (7) together with condensed water of

primary vapour (18) at outlet of HC flow into the LC which

heat rejection takes place. The secondary vapour directly and

the condensed water through refrigerant valve (18-19) come

into LC. At the outlet of LG the strong solution (4) is produced,

which through heat exchanger I and the solution reducing

valve (4-5-6) flows into absorber. Refrigerant liquid leaving

LC (8) which is sum of refrigerant originating from LG and

HC, on refrigerant expansion valve, continues to the

evaporator (8-9) where it is evaporated in low pressure.

Vaporization in evaporator occurs by extracting the heat from

medium and it leads to the medium being cooled. The vapour

refrigerant from evaporator goes into absorber and is absorbed

by the strong solution and the mentioned process is repeated.

Figure 1. P-T diagram for a half-effect absorption cooling system [22].



Figure 2. A solar assisted single LiBr-H2O absorption cycle [24].

Figure 3. P-T diagram for a series double-effect absorption cooling system. [22].

b) Parallel Double Effect Absorption Refrigeration

Systems

R. Maryami et al. [22] presented the schematic illustration

of parallel double effect absorption refrigeration cycle as

shown in Figure 4. The cycle consists of absorber, evaporator,

two condensers, two refrigerant expansion valves, two

solution reducing valves, two generators, two solution pumps

and two solution heat exchangers. Similar to the series

double effect absorption refrigeration system, the

components of parallel double effect cycle operate at three


pressure levels, i.e. lower, medium and high level pressures.

Parallel and series double effect cycle description is

approximately identical and the process in parallel double

effect system is as follows. At an intermediate temperature

(i.e. the temperature of HG), waste heat (i.e. the output heat

from an industrial process), is added to the HG to concentrate

the weak absorbent solution flowing from the absorber (13),

for partially vapour rizing working fluid. Note, the weak

absorbent solution in HG (13) is at high pressure side and

from absorber at lower pressure side (1) comes into HG

through the solution heat exchanger I, LG and solution heat

exchanger II using pumps (1-2-3-11-1213). In the LG, the

weak absorbent solution (3 and 11) is at medium pressure

level. At outlet of HG, the strong absorbent solution (14), to

be supplied to the absorber, is generated. The strong

absorbent solution goes into absorber through the solution

reducing valves, LG, solution heat exchangers I and II (14-

15-16-4-5-6). The vaporized working fluid at outlet of HG

(17) flows to the HC, where heat at a relatively high

temperature is delivered to LG in order to produce secondary

vapour (7), weak absorbent solution (11) and strong

absorbent solution (4) at the outlets of LG and also

condensed water at outlet of HC (18). Subsequently, from the

HC and LG components, the liquid of primary vapour and

secondary vapour go into the LC respectively which is

maintained to a medium pressure. In the outlet of LC

refrigerant fluid is produced at a intermediate temperature

which flows into evaporator through refrigerant expansion

valve (8-9). In the evaporator the working fluid is evaporated

by using waste heat at the low temperature. The vaporized

working fluid (10) flows to the absorber, where it is absorbed

by the strong absorbent solution (6) which is coming from

the generator. Because of absorption, the useful heat, at an

intermediate temperature, is delivered. Finally, the weak

absorbent solution is returned to the generator through the

pumps.

iv. Triple Effect Absorption Refrigeration Systems

R. Maryami et al. [22] showed a schematic diagram of a

triple effect absorption refrigeration system in Figure 5. It

consists of three generators: HG, middle generator (MG)

and LG. It also consists of three condensers: HC, middle

condenser (MC) and LC which from LC heat is rejected to

the surrounding while from HC and MC heat is delivered to

MG and LG respectively. This absorption refrigeration

system implies four pressure levels. The HG and HC

operate at high pressure. The MG and the MC operate at

pressure related to temperature of saturated liquid extracted

from MC (18) while LG and LC operate at middle pressure.

The evaporator and the absorber work at low pressure. In

this cycle, the weak absorbent solution (1) is pumped from

the absorber to the HG through LG and MG and three

solution heat exchangers I, II and III (1-2-3-11-12-13-21-

22-23). In the HG, similar to double effect systems the

solution is heated at high temperature to boil out the

refrigerant vapour from the solution. The primary vapour

released from HG (27) enters HC and condenses. The

strong solution leaving HG (24) flows into the GM through

the solution heat exchanger III, where solution is cooled to

some extent by exchanging heat with the weak solution (13).

In the MG some more vapour (17) is released that flows to

MC. The stronger absorbent solution which is now leaving

MG (14) then goes to LG through solution heat exchanger

II where the solution is further cooled by exchanging heat

with the weak solution (3). Thus, more refrigerant is

produced in LG that enters the LC (7) and heat is released

to the sink. The total refrigerant entering LC is the sum of

all those coming from the three generators. The liquid

refrigerant from LC (8) flows into the evaporator through a

refrigerant expansion valve which vaporizes after absorbing

heat from the medium to be cooled. Refrigerant vapour (10)

then goes to the absorber to be absorbed by the strongest

solution coming from the LG (6) through the solution heat

exchanger I and solution reducing valve. The cycle then

gets completed and started again.

Figure 4. P-T diagram for a parallel double-effect absorption cooling

system. [22].

v. Solar Open Absorption Cooling Systems

Kaushik et al. [20] explained that in the open cycle

absorption cooling system, one can have two possible options

depending on whether the absorption system is made open

from the regenerator part or from the evaporator part. The

first option is the open regeneration absorption cooling

system, where the system is made open to the regenerator

side. The second option is the open evaporative cooling

system, in which the system is open to the evaporator part.

These two systems are discussed in the following sections.

a) The Solar Open Regenerative Absorption Refrigeration

System

Hassan et al. [5] presented this type of open absorption

cooling system which is open to the regenerator side. A

schematic diagram of the solar-operated open absorption

refrigeration system is shown in Figure 6. In this type of



cooling machines, the dilute absorbent solution is heated and

subsequently re-concentrated in the solar collector, which is

open to the atmosphere, due to the evaporating process. The

strong regenerated solution leaves the collector and passes

through a liquid column, to allow the strong solution to drop

from atmospheric pressure to the reduced evaporator pressure.

The strong solution then goes to the absorber after passing

through a regenerative heat exchanger. In the absorber, the

strong solution absorbs water vapour from the evaporator and

therefore the latent heat of vaporization of the refrigerant is

taken allowing a low temperature in the evaporator space by

a vapour absorption process. The resultant weak solution is

pumped from the absorber back to atmospheric pressure

through the regenerative heat exchanger then through the

regenerator to completing the cycle.

Figure 5. P-T diagram for a triple-effect absorption cooling system. [22].

b) The Solar Open Evaporative Absorption Refrigeration

System

Hassan et al. [5] identified the main difference with the

other cycles is the absence of condenser. In an open

absorption cooling system, flows of heat and mass take place

to and from the system at relatively low temperatures of the

heat source. The entire operation takes place at atmospheric

pressure, thus eliminating the need for vacuum vessels. Water

is generally used as the refrigerant in this system. The reason

is that the refrigerant is released to the ambient atmosphere

and therefore it should be environmentally friendly. Another

reason is the requirement of a continuous supply of make-up

refrigerant during the entire operating time. As a

consequence, water as a refrigerant is very suitable working

fluid in the open cycle absorption cooling system. Absorbents

like LiBr, LiCl or CaCl2 are usually used with water

refrigerant.

Figure 6. The solar powered open regenerative absorption refrigeration

system. [25].

vi. The Diffusion Absorption Refrigeration Systems

Two Swedish engineers, Platen and Munters, developed a

special kind of absorption cooling cycle in 1922. The cycle is

so called as Platen–Munters cycle and the machine is called

the diffusion absorption refrigeration (DAR) system. The main

difference between the other cycles and the DAR machine is

the lack of pumps or any moving parts that require auxiliary

energy supply. Therefore, the system is noiseless and free from

vibration. Moreover, it exhibits good reliability, durability, and

minimum maintenance costs [26]. The whole unit has the same

total pressure and as a consequence, the usual throttling

required for the pressure reduction is removed and the solution

pump can be a simple gas bubble thermally powered pump.

Figure 7 shows a schematic diagram for the DAR machine and

its main components. The main components of the DAR


system are the generator, condenser, evaporator, absorber,

rectifier, and a bubble pump that is besides the auxiliary gas

heat exchanger (AGHX) and solution heat exchanger (SHX).

The cycle uses a three-component working fluid consisting of

the refrigerant, the absorbent, and the auxiliary gas. Ammonia

is generally used as a refrigerant, water is used as an

absorption media and the auxiliary gas is a kind of inert non

condensable and non-absorbable gas like hydrogen or helium.

The principle of the cycle operation is similar to the single

stage absorption cycle. The difference is that the total pressure

is the same in the entire system. Ammonia circulates through

all the system components. Water–ammonia solution circulates

through the generator, gas bubble pump, and absorber. The

ammonia–hydrogen gas mixture circulates in the auxiliary gas

circuit which includes the evaporator, absorber, and gas heat

exchanger (not shown in figure), and is driven by natural

convection. The generator temperature varies typically

between 120 and 180 °C, and the practical COP varies

between 0.2 and 0.3 with cooling capacity of 25–100 W.

However, large capacity systems are not considered as

attractive.

Figure 7. The diffusion absorption solar refrigeration system. [25].

vii. Dehumidifier–Evaporator–Regenerator (DER) System

Hellmann and Grossman [27] reported an investigation of

an evaporative cooling open cycle absorption heat pump that

is capable of utilizing low grade heat sources such as solar

heat as its source of power was by. The system, referred to as

dehumidifier–evaporator–regenerator (DER) cycle

represented in Figure 8, operates at atmospheric pressure and

its immediate application is for cooling and air conditioning.

Figure 8. The dehumidifier–evaporator–regenerator (DER) system [25].

2.1.2. The Intermittent Solar Absorption Refrigeration

System

According to Trombe and Foex [28], most of the earlier

solar absorption chillers were manufactured for intermittent

operation, however researchers later focused over continuous

operation chillers because of low coefficient of performance

of the intermittent chillers.

i. he Single Stage Intermittent Solar Absorption

Refrigeration System

A schematic diagram of the basic single effect intermittent

solar absorption cycle is shown in Figure 9. The working

principle of this system was presented for the first time by

Venkatesh and Mani [29]. Staicovici [30] described an

intermittent single-stage H2O/NH3 solar absorption system of

46 MJ/cycle. He used evacuated solar collectors with

selective surfaces to heat the generator.



Figure 9. The single stage intermittent solar absorption cooling system. [25].

ii. The Two Stage Intermittent Solar Refrigeration System

Pridasawas W. et al. [31] presented the two-stage

intermittent as a system that can be used to improve the

operation and enhance the efficiency of the single stage

system. Moreover, it can solve the limiting operation

temperature in the single stage system. In the two stage

system there are two levels of generator pressures, a high-

pressure generator (HPG) and a low-pressure generator

(LPG). The HPG and LPG generators are sets of flat plate

collectors. The generators are built into the flat plate solar

collector. As shown in Figure 10, there are two weak solution

storage tanks (Tank 1 and Tank 2) to store and supply to HPG

and one tank (Tank 3) to store and supply the weak solution

to LPG. Tank 1 and Tank 2 do not supply the weak solution

to HPG at the same time; they alternate daily. Tank 3 is

operated every day to supply the weak solution to Tank 1 or

Tank 2, whichever is not supplying solution to HPG.

Figure 10. Construction of the two stage intermittent solar absorption cooling system. [25].

2.1.3. Hybrid Solar Absorption Refrigeration Systems

Kaushik and Yadav [21] presented two designs for a hybrid

double-absorption solar cooling system. These are: a

conventional closed-cycle and open-cycle absorption systems

with an additional open-absorber component through which

the process room air is passed, cooled and dehumidified. The

cooling produced in the evaporator is utilized to remove heat

from the open absorber and the process air being circulated.

A comparative study of the open and closed cycle options has

been made. They concluded that the hybrid double-

absorption solar cooling systems are better in performance

than conventional systems and an open-cycle double-

absorption system is even more attractive and cost effective

as compared to closed-cycle option.

2.2. Solar Adsorption Refrigeration Technologies

Vapour adsorption technology was introduced for the first

time in 1848 by Faraday, using a solid adsorbent. Adsorption


cycles were first used in refrigeration and heat pumps in the

early 1990s. The disadvantages of liquid–vapour systems

were overcome by using solid–vapour cycles; this technology

was first marketed in the 1920s [32]. Adsorption–

refrigeration systems are environmentally friendly

alternatives to the other existing systems, since they can use

refrigerants that do not contribute to ozone layer depletion

and global warming. They also operate at lower costs in

comparison with the absorption systems, as they do not need

solution pump. The refrigeration as solar energy application

is particularly attractive because of non-dependence on

conventional power. Adsorption refrigeration technology are

used for many specific applications, such as purification,

separation and thermal refrigeration technologies [33]. An

adsorption cooling system is not only advantageous for its

zero ODP (ozone depleting potential) but also for other

positive features [34, 35]: K. R. Ullah et al. [15] verified that

adsorption technology can accommodate high temperature

heat sources (over 500V) without corrosion, whereas

corrosion occurs above 200°C in absorption technology.

Adsorption technology is better equipped to handle vibration

issues in a cooling system than absorption technology.

Because of the liquid absorbent present in an absorption

system, vibrations can cause serious damages, such as flows

from the absorber to condenser or from the generator to

evaporator, potentially polluting the refrigerant. Adsorption is

immune to this condition, and can thus be used in

locomotives and fishing boats. An adsorption system is

simpler to design than an absorption system.

2.2.1. Working Principle of Solar Adsorption Cooling

Systems

The adsorption process differs from the absorption process

in that absorption is a volumetric phenomenon, whereas

adsorption is a surface phenomenon. The primary component

of an adsorption system is a solid porous surface with a large

surface area and a large adsorptive capacity. Initially, this

surface remains unsaturated. When a vapour molecule

contacts the surface, an interaction occurs between the

surface and the molecules and the molecules are adsorbed on

to the surface [25]. Adsorption is a process in which

molecules of a fluid are attached to a surface. The surface is

composed of a solid material. The molecules do not perform

any chemical reaction; they merely discard energy when

attached to the surface. The phase change (from fluid to

adsorbate) is exothermic, and the process is fully reversible

[36]. Figure11 presented a solar powered continuous

adsorption refrigeration system.

Figure 11. Schematic diagram of the solar powered continuous adsorption–refrigeration system (1) condenser, (2) ammonia tank, (3) expansion valve

evaporator. [37].

2.2.2. Adsorbents and Working Pairs

Wang LW et al. [34] noticed that the most common

working pairs used in adsorption technologies are silica gel-

water, activated-carbon-methanol, activated carbon-ammonia,

zeolite-water, activated-carbon granular and fibber adsorbent.

In an adsorption refrigeration technique, the working pair

plays a vital role for optimal performance of the system. The

best performance is achieved if the adsorbent demonstrates

the following characteristics: A large adsorption ability; the

ability to change capacity with the variation of temperature; a

flatter isotherm; excellent compatibility with the refrigerant.

In addition, the refrigerant should also demonstrate certain

characteristics for better performance, such as a significant

latent heat capacity, exact freezing point and saturation

vapour pressure, non-flammability, good thermal stability,

anti-toxicity, lack of corruption/adulteration, etc.

Unfortunately, there are few working pairs that completely

fulfill the requirements discussed above. However, there are

some working pairs that come close such as: [34, 35, 38]:

1. silica gel/water,

2. activated-carbon/methanol,

3. activated-carbon/ammonia,

4. zeolite/water,

5. activated carbon granular and fiber adsorbent,

6. metal chloride/ammonia,

7. composite adsorbent/ammonia,

8. metal hydrides and hydrogen,

9. metal oxides and oxygen.

2.2.3. Solar Physiorption Refrigeration Systems

According to Wang DC et al. [35], in physisorption

technology, the adsorbate molecules form a van der Waals

interaction with surface molecules within a vacuum and clean



environment instead of forming a chemical bond.

2.2.4. Solar Chemisorption Refrigeration Systems

Hassan HZ et al. [5] defined a chemisorption as a process

in which the adsorbate and the adsorbent atoms form a

complex compound by sharing their electrons in a chemical

reaction process. A valence force is created that is stronger

than the van der Waals interaction described in the

physisorption process. To free the adsorbate, a very high heat

of up to 800 kJ/mole is required. It is a specific process that

is suitable for a fixed gas and a fixed solid adsorbent.

Moreover, it is a monovariant system, where physisorption is

a divariant technique. One of the main advantage of

chemisorption technology is a high COP [39]. A complete

cycle of chemisorption technology consists of four

consecutive processes: desorption, condensation, evaporation

and adsorption [34].

3. Exergy Analysis of Solar Refrigeration

Technologies

3.1. The Necessity of Exergy Analysis of a System

The working performance of refrigeration systems can be

investigated based on the energy and exergy approaches. The

former is obtained from the first law of thermodynamics,

accounting the quantity of energy in the entire system. While

the latter is based on the second law of thermodynamics

which involves irreversibility and represents quality of

energy in a system. The exergy based analysis is very

important to indicate the cause, location and magnitude of the

system inefficiencies and provides the true measure how a

system approaches to the ideal [40, 41]. Many researchers

propose also that the thermodynamic performance of a

process is best evaluated using exergy analysis. Exergy

analysis can be used to improve the efficiency of a system

after determining the sources and the magnitude of

irreversibility. Muhammad Faisal Hasan et al. [42] who did

the exergy analysis of Serpentine Thermosyphon Solar Water

Heater, predicted the performance of these type of systems

by a summary of exergy destruction and loss, dimensionless

exergy and exergy destruction ratio. Soteris A. Kalogirou et

al. [43] presented a review on exergy analysis of solar

thermal collectors and processes. R. Maryami and Dehghan

[22] presented an exergetic analysis of all system’s

components after their energy analysis. The half effect, single

effect, series class of double effect, parallel class of double

effect and triple effect absorption refrigeration cycles were

compared together in this study. The exergetic evaluation of

the thermodynamic flows, which goes through this cycle, was

performed for a refrigerating capacity 300 kW. According to

energetic and exergetic analysis, the results showed that the

coefficient of performance and exergy efficiency increases

from the half effect, to the single, double and triple effect

refrigeration system while total exergy change approximately

decreases. Golchoobian et al. [44] investigated exergy

analysis of solar ejector refrigeration system (using R141) for

air conditioning of building in Tehran (Iran). A significant

improvement in exergetic performance of the system was

observed using hot water storage tank. Higher exergy losses

were also observed in collector followed by ejector. Overall,

system irreversibility was found more during first and last

working hours. G. Gutiérrez-Urueta et al. [45] conducted a

study on the energy and exergy analysis of water-LiBr

absorption systems with adiabatic absorbers for heating and

cooling. Their results show that the coefficient of

performance and the exergy efficiency were influenced by

the temperature, these parameters increase with the increase

of the absorber temperature. Sanju Thomas et al. [46]

analyzed and identified the potential of using a standalone

solar thermal energy generation unit for power, heating and

cooling applications with some modifications in the LFR.

Comparisons of adsorption and absorption methods are

analyzed in the application of refrigeration. Energy, entropy

and exergy analysis were done for the major components in

solar thermal based polygeneration system. Vadiee A. and M.

Yaghoubi [47] demonstrated that beside thermal energy

analysis, an exergy analysis can lead to understand better the

energy conservation potential. Many studies have been done

on exergy analysis of solar cooling systems. The last review

in this domain is that of M. U. Ahmadi P. et al. [48]. They

conducted an exergy efficiency analysis on multigenerational

energy system which generates power, water and cooling

simultaneously. Siddiqui et al. [49], focused in their review

on different solar powered absorption refrigeration systems,

that is: diffusion absorption systems, ejector based absorption

systems, compression absorption systems and

cogeneration/trigeneration absorption systems. The

thermodynamic properties of most common working fluids

as well as use of ternary mixtures in solar powered

absorption systems have been reviewed in this study. K. R.

Ullah et al. [15] conducted a review on solar thermal

refrigeration and cooling methods, the absorption and

adsorption cycles were presented. Gebreslassie et al. [50]

performed an exergy analysis for a lithium-bromide/water

pair in a multiple-effect absorption system. They conducted a

comparative exergy study among single-effect, double-effect

and triple-effect absorption for unavoidable destruction. Sunil

Kumar Sansaniwal et al. [40] presented a comprehensive

review on energy and exergy analyses of various typical solar

energy applications, they concluded that exergy efficiencies

are highly dependent on the intensity of solar radiations on

daily basis. Esa Dube Kerme et al. [51] analyzed the effects

of generator inlet temperature, effectiveness of the solution

heat exchanger and the pump mass flow rate on the energetic

and exergetic performance of the chiller system. The

examined performance parameters were coefficient of

performance, exergetic efficiency, exergy destruction, fuel

depletion ratio and improvement potential.

3.2. Chronological and Statistical Works on Exergy

Analysis

Mr. Manish G. Vasava et al. [52] established a

chronological and statistical work about the development


scenario of exergy analysis in the world. In table 1, it is

found that, exergy analysis has grown from child to adult

from 1824 to till 2014 and scientists and researchers are

enjoying the fruitful results of expertise in the field of Exergy

analysis. In table 2 another chronological and statistical work

have been done about the different fields of applications of

exergy, it is clear that, since its development, exergy analysis

has been widely used in most of the engineering applications.

The study of exergy analysis related to VCRS has been

increased in last decade only. It’s can also be noted from

those two tables that the fields of Cryogenics and Chemical

Processes are very concerned by studies on exergy analysis

before 2014.

Table 1. Development Scenario of Exergy [52].

Sub Group Duration of Year Number of Research Papers Published

Conceptual Literature in line of Exergy 1824-1952 33

Inception of Exergy 1953 1

Development of Exergy analysis 1954-1970 71

Expertise Phase of Exergy After 1970 292

Table 2. Application of Exergy Analysis [52].

Type of Engineering Applications of exergy

analysis

Number of Research Papers Published

1931-1960 1961-1980 1981-2000 2001-2010 2011-2014

Steam Power Cycles 2 4 5 5 -

Gas Turbine Cycles - 4 6 9 -

Renewable Energy Cycles - - 4 1 -

Combined & Cogeneration Cycles - 9 20 11 -

Heat exchangers & Heat Networking 1 5 18 8 -

Cryogenics 4 15 14 12 -

Chemical Processes 3 10 15 11 -

Distillation & Desalination 1 6 15 8 -

Industrial & Agricultural Systems 2 6 22 8 -

Environmental Appli. - 8 21 12 -

VCRS - - - 5 8

3.3. Mathematical Equations for Exergy Analysis

3.3.1. Exergy

The exergy is defined as the maximum possible reversible

work that can be produced by a stream or system in bringing

the state of the system with a reference environment. Exergy

is conserved in an ideal process (except for those developing

work) and destroyed during a real process. However, quality

of energy is more important to identify work potential of

system and is expressed by term ‘exergy’. When exergy

losses its quality in a process, it gets destroyed and thus

affects the overall work performance. Therefore, energy is

conserved while exergy is accumulated. As compared to low

exergy content of energy, high exergy content is more

important to obtain desirable performance of the system [53].

Therefore, the optimum design, resource utilization,

sustainability and impact of the environment on energy may

be investigated through exergetic analysis. Moreover, exergy

analysis is helpful in assessing and comparing the various

processes and systems meaningfully and rationally [18, 54].

Blanco-Marigorta et al. [55] identified the location,

magnitude and the thermodynamic inefficiencies in a solar

thermal power plant using exergy analysis. Exergy

calculation is based on the second law of thermodynamics

which deals with quantity and quality of energy relative to

the reference state. Energy and exergy efficiencies are also

known as first and second law efficiency, respectively. In

general, exergy efficiency is often calculated to be lower than

that of energy efficiency due to the involvement of

irreversibilities caused by thermodynamics imperfections

during the process [41, 56].

Many kinds of exergy are existing due to the presence of

various random activities such as macroscopic forms of

energy, turbulence flow of molecules and the concentration

of species with respect to dead state [57]. Exergy

performance of a system included exergetic efficiency,

exergy losses, exergy improvement, coefficient of

performance and exergetic coefficient of performance.

Neglecting nuclear, magnetic and electric effects, the

exergy for a specific state with reference to the environment

can be written as: [45]

. .

0 0 0( ) ( )E m h h T s s= − − − (1)

where h0 and s0 are evaluated at the reference environment

temperature T0=293.15 K and atmospheric pressure. The

subsequent analysis uses temperatures in K.

Zhong Ge et al. [58] conducted a study on exergy analysis

of a flat plate collector. The model was experimentally

validated and the results show that the exergy performance

could be significantly improved by adjusting the operating

parameters which are solar irradiance, ambient temperature,

fluid inlet temperature and fluid mass flow rate. One cannot

undertake the exergy analysis of a solar system without

taking into consideration the solar collector. Esa Dube Kerme

et al. [51] examined the influence of the solar collector types

on the collector efficiency and the useful heat gain by the

collector for the best performance:



,0

,

. . .ln( )col out

u u col pcol in

TE Q m c T

T= − (2)

Zhong Ge et al. [58] proposed a micro unit dA taken from

the absorber plate in steady state according to the first law of

thermodynamics:

[ ( ]u s l l p aQ Q Q S U T T dA∂ = ∂ − ∂ = − − (3)

where Qu is the useful heat rate gain of the flat plate solar

collector, Qs is the radiation flux absorbed by the absorption

plate, Ql is the heat loss from the solar collector to the

environment, S is the radiation flux absorbed by a unit area

of the absorber plate, Ul is the local heat loss coefficient of

dA, Tp is the temperature of dA and Ta is the ambient

temperature.

According to the integral mean value theorem, the

following can be deduced from equation (3):

[ ( )] [ ( )]u l p a p l pAp

Q S U T T dA A SU T T= − − = −∫∫ (4)

where A is the area of the absorber plate, lU is the integral

average of Ul over the absorber plate, and pT is the integral

average of Tp over the absorber plate.

Taking a micro unit dy from the absorber plate, Zhong et al.

[58] obtained the heat transferred from the absorber plate to

the glass comprises radiative heat and convective heat. The

radiative heat transfer from the absorber plate to the glass of

dy is follows:

4 41

, 1

1

1

( )

1 11

p

r pp

p

T Tq

w w w

σε ε

ε ε

−−

∂ =− −+ +

(5)

The convective heat transfer coefficient between the

absorber plate and the glass is calculated from the Nusselt

number according to the following equation [58]:

1 11.6

3 31708 1708 (sin1.8 ) 1708 cos cos1 1.44 1 (1 ) / 2 1 1 1 / 2

cos cos cos 5830 5830u

Ra RaN

Ra Ra Ra

θ θ θθ θ θ

= + − + − − + − + −

(6)

where

31( )pg d T T

RaV

βα

−= (7)

and

, 1u air

c p

N Kh

d− = (8)

According to Evangelos Bellos et al. [59] the available

solar irradiation in the collector level is calculated as follow:

.s c TQ A G= (9)

The useful heat production of the collector field is

calculated by the exergy balance on the fluid volume [58]:

, ,( )u c p col out col inQ m c T T= − (10)

The thermal efficiency of the collector is the ratio of the

useful heat to the available solar radiation on the collector

level. It is given as follow [58]:

uth

s

Q

Qη = (11)

The mass flow rate in the collector field is calculated

according to the specific mass flow rate [58] as it is presented

below:

.

0.02col cm A= (12)

The solar collector performance:

.

,

x

ex sysel sol

E

P Eη =

+ (13)

The exergy collected from the collector’s bottom is given

by:

0(1 )QB

TQ

Tε = − (14)

where Q is the heat extracted that is deliver to the user, T0 is

the temperature of the dead state and εQ is the exergy

extracted from the collector.

The coefficient of performance of a refrigeration system is

given by [45]:

.

. .

evcool

elg

QCOP

Q W

=+

(15)

The exergetic coefficient of performance of the ejector -

absorption heat transformer is as follow [60]:


.

.

0

... .

0 01 2

(1 )

(1 ) (1 )

aa

p e p eg eg e

Tq

TECOP

T Tq q W W

T T

−=

− + − + +

(16)

3.3.2. Exergetic Efficiency

G. Gutiérrez-Urueta et al. [45] investigated and concluded

that the exergy efficiency of absorber and solution heat

exchanger increases as the absorption temperature increases,

but it shows a decreasing behaviour when generation

temperatures increases. The results obtained allow also the

identification of parameters that may influence the exergy

efficiency of the adiabatic absorption system. In general,

exergy efficiency is often calculated to be lower than that of

energy efficiency due to the involvement of irreversibilities

caused by thermodynamics imperfections during the process

[18, 56]. Baral et al. [61] investigated the energy and

exergetic performance of a small scale solar assisted organic

Rankine cycle (ORC) comprised of scroll expander, heat

exchanger and reciprocating feed pump for electricity

generation. The influence of various process parameters

(evaporating and condensing pressure, the degree of

superheating, pressure ratio, expander inlet and dead state

temperature) on energy and exergy efficiencies of the system

was studied. The overall thermal and exergy efficiencies of

the system were obtained to be 7.5% and 43.7%, respectively.

Joshi et al. [62] investigated the working performance of

hybrid PV/T system in terms of energy and exergy

efficiencies and improvement potential factors. The energy

and exergy efficiencies of hybrid PV/ T system were found to

be 45.0% and 16.0%, respectively. Maryami and Dehghan

[22] presented variations of exergetic efficiency based on the

generator temperature for five configurations of absorption

refrigeration systems at different condenser and evaporate

temperatures. For all configurations (half to triple effect

absorption refrigeration system) and each evaporator and

condenser temperature, the exergetic efficiency increases up

to a certain generator temperature and then decreases. In fact,

the results show that for a given evaporator and condenser

temperature, there was a minimum generator temperature

which corresponds to maximum exergetic efficiency. It was

considered that the exergetic efficiencies reach a maximum

value at slightly lower generator temperatures than the COP.

Furthermore, at the higher generator temperatures, while

COP remains approximately constant for all absorption

systems, exergetic efficiency decreases gradually for each

value of evaporator and condenser operating temperature

considered. The possible reason for this was that, when

increasing the generator temperature negatively affects the

exergetic efficiency value. Unlike the COP variation with

evaporator and condenser temperatures in a certain generator

temperature, it can be seen that the better exergetic efficiency

of the system is achieved at lower condenser temperature and

lower evaporator temperatures within the same generator

temperature. Otherwise, exergetic efficiency decreases by

increasing condenser and evaporator temperatures for a given

generator temperature. Kaushik and Arora [63] achieved the

1st and 2nd law thermodynamic analysis of single effect and

double effect LiBr–water system which was connected in

series. Their 1st law analysis results indicate that the COP of

series flow double effect system was 60 to 70% greater than

the single effect system where the optimum COP was

reached at 91°C for single effect and 150°C for double effect

system. Similarly their 2nd law analysis results indicate that

the optimum exergetic efficiency was reached at 80°C for

single effect and 130°C for double effect system. Reddy et al

[64] presented the exergetic performance of 50 MW solar

power plant having parabolic dish Stirling engine and

detected that the energy and exergy efficiencies of whole

system were obtained to be 77.8% and 32.0%, respectively.

Izquierdo et al. [65] concluded after their study that the

double stage system has about 22% less exergetic efficiency

than the single effect one and 32% less exergetic efficiency

than the double effect one. In another study, Izquierdo et al.

(2005) [66] observed that the exergetic efficiency of the

double stage cycle considered was lower than that of the

single and double effect cycles because it worked with heat at

lower temperature. The double stage system had about 22%

less exergetic efficiency than the single effect one and 32%

less exergetic efficiency than the double effect one.

Kuzgunkaya and Hepbasli [67] obtained that the exergy

efficiencies of a ground source heat pump (GSHP) and whole

system were reported to be 21.1% and 15.5%, respectively.

Yildiz and Ersoz [68] studied the simulation and

experimentation for aquaammonia based diffusion absorption

system with helium as the inert gas indicated that the energy

efficiency was reported to be equal to 0.1858 whereas the

exergy efficiency was reported as experimentally found to be

0.0356. Lostec et al. [69] implemented the second law

optimization of an absorption refrigeration system. Their

results indicated the presence of three optimum values of

COP 0.56, 0.62 and 0.69 for the absorption system that

minimizes UA value, irreversibility and exergetic efficiency

respectively. R. Maryami et al. [22] give the variation range

of maximum COP and exergetic efficiency and minimum

total exergy change of some usual cycles in Table 3.

Gunerhan and Hepbasli [70] presented the performance

evaluation and exergetic modelling of solar water heating for

residential applications. The exergy efficiencies of solar

collector and whole system were reported to vary from 2.02%

to 3.37% and 3.27–4.39%, respectively.

The exergy efficiency is given as follows [40]:

..

. .1

out d

in in

ExEx

Ex Ex

ε = = − (17)



Table 3. Variation range of maximum COP and exergetic efficiency and minimum total exergy change of the cycles. [22].

Cycle COP [%] Exergetic efficiency [%] Total exergy change [kW]

Half effect 0.407–0.438 9.781–18.28 59.62–67.13

Single effect 0.768–0.825 13.43–24.03 41.20–49.58

Series double effect 1.217–1.397 14.19–24.50 41.00–55.46

Parallel double effect 1.306–1.440 15.33–23.63 44.28–54.95

Triple effect 1.482–1.905 14.21–24.09 65.06–232.1

Exergy efficiency is given by Sunil et al as [40]:

00

273(1 ) (1 )

293th

TT

Tε η β η += − ∆ + −

+ ∆ (18)

The overall exergy efficiency of a system is given by [40]:

.

.1 ( )loss d

sun

E x Ex

E x

ε += − (19)

3.3.3. Exergy Losses

Another words to express exergy loss is exergy

destruction or irreversibility. Reddy et al. [64] presented the

exergetic performance of 50 MW solar power plant having

parabolic dish Stirling engine. The energetic and exergetic

evaluation of each component (parabolic dish collector,

Stirling engine, heat exchanger and cooling tower) were

realised. The major energetic losses of 42.3 kW were

observed in Stirling engine followed by collector receiver

(19.4 kW). While the highest exergetic losses of 32.9 kW

were reported in the collector receiver followed by Stirling

engine (22.1 kW)[57]. Esa Dube Kerme et al. [51] also

indicated that the main source of the exergy destruction is

the solar collector. In the solar collector, 71.9% of the input

exergy was destroyed which accounted for 84% of the total

exergy loss. Additionally, 7.1% of the inlet exergy was lost

in the generator which was equivalent to 8.3% of the total

exergy loss. The overall exergetic improvement potential of

the system was approximately 84.7%. Kilic and Kaynakli.

[71] performed the 1st and 2nd law thermodynamic analysis

of LiBr–water absorption chiller. Their results indicated

that irrespective of the working conditions, the generator

presents the highest exergy loss component i.e. 45.6% of

the overall exergy loss in the chiller while the pump and

refrigerant heat exchanger represents the lowest exergy loss

component in the chiller. Therefore, exergy loss (exergy

waste and exergy destruction) indicates possible

improvement in overall performance of the system. [40]

Lostec et al. [69] results showed that 33% and 34% of the

exergy destruction takes place inside the absorber and

desorber of the absorption system respectively. Onan et al.

[72] utilized MATLab software to simulate the hourly

performance of a 106 kW system in an environment where

the ambient temperature varies from 40.3°C to 13.2°C.

Their results indicated that the total exergy loss in the

collector ranges from10 to 70% which is the maximum

compared to the other components of the solar powered

LiBr–water system. Yildiz and Ersoz. [68] performed both

the simulation and experimentation for aquaammonia based

diffusion absorption system with helium as the inert gas. It

was reported that the highest energy and exergy losses

occurs in the solution heat exchanger which was

experimentally determined to be 44.4% and 64%

respectively of the total energy and exergy losses in the

system. They also reported that the lowest exergy loss

occurs in the condenser. Ahamed et al. [73] demonstrated

that exergy losses in the components decrease with the

increase of evaporating temperature. The higher the

temperature differences in any component with the

surroundings, the higher the exergy loss. Arora and Kaushik.

[74] explained that much research is done to find the exergy

destruction in the different components of the vapour

components with different refrigerants. The efficiency

defects in the compressor, condenser, throttle vale and

evaporator. While investigating the working performance of

a hybrid PV/T system in terms of energy and exergy

efficiencies and improvement potential factors, Joshi et al.

[62] obtained exergetic improvement potential of 250–610

W, corresponding to the exergy destructions.

The exergy destruction [45]:

. .. . . .

0

1 1

(1 )

n n

k kkj kjj k k

TI Q m E m E W

T = =

= − + − −

∑ ∑ ∑ (20)

3.3.4. Exergy Improvement

The exergy improvement is an important criterion in

optimum designing of solar systems and can be achieved

through minimizing exergy losses or irreversibilities occurred

in the process [40]. Kuzgunkaya and Hepbasli [67]

experimentally studied the exergetic performance of a ground

source heat pump (GSHP) dryer for drying laurel leaves. The

maximum potential of exergetic improvement was calculated

to be 551.63 W by using expression given by [75]. Recently,

Fudholi et al. [75] also investigated energy and exergy

analyses of a hybrid solar drying system for drying silver

jewfish. The collector and dryer efficiency were obtained to

be 41.0% and 23.0%, respectively (for mass flow rate of

0.0778 kg/s and solar radiation of 540 W/m2). The average

exergetic improvement potential of 236 W was also

calculated [40].

The exergy improvement potential is as follow [40]:

.

(1 ) dIP Exε= − (21)

3.3.5. Exergetic Coefficient of Performance

The overall exergy input to the solar powered absorption

cooling system is the exergy of the solar radiation falling on


the solar collector, and it is the function of the sun’s outer

surface temperature (Ts=6000 K) and defined as [76]:

0

4.

0,

1 41

3 3x in

c Ts s

TE

T T

TA I

= + −

(22)

The irreversibility ratio of each component of solar driven

absorption cooling system is defined as the ratio of exergy

destroyed in each component to that of the total exergy loss

of the system and it is given by [77]:

.

.

,

xd

xd total

EIR

E

= (23)

Aman et al. [78] observed more irreversibility in absorber

(63%) followed by generator (13%) and condenser (11%).

The value of COP of the system was obtained to be 1.86,

however, this value was reported to decrease (up to 0.60) due

to exergy degradation of all components of the system. The

overall efficiency of cooling system was found lower as

compared to LiBr-H2O based cooling and thus latter was

recommended to achieve the higher exergetic performance.

The exergy efficiency of different solar power systems (solar

power tower, solar thermal power plant, solar gas power

turbine, dish/engine, Kalina, Rankine and Brayton cycle) are

observed to vary from 6.0–43.7%. The highest exergy

destructions are caused by heliostat field, receiver and

evaporator. Also, the waste heat from different power cycles

can be utilized in combination with high grade energy for

performance enhancements.

R. Gomri et al. [79] presented in Figures 12 and 13 the

relation between exergy efficiency, COP and the temperature

of HPG and LPG in a double effect absorption refrigeration

system. These figures can be taken as a model for the

variation of the coefficient of performance and the exergy

efficiency of a system with the temperature of the generators

in a double effect refrigerator.

Figure 12. Coefficient of performance (COP) of double effect absorption refrigeration system versus HPG and LPG temperatures. [79].

Figure 13. Exergy efficiency of double effect absorption refrigeration system versus HPG and LPG temperatures [79].

They also presented a T-s diagram of a LiBr/water double

effect absorption refrigeration cycle in Figure 14. This

diagram is particularly interesting in interpreting and also for

a better comprehension of the exergy analysis of a

refrigeration cycle.



Figure 14. LiBr/water double effect absorption refrigeration cycle on T–s diagram. [79].

The maximum improvement in the exergy efficiency for a

system is achieved when the exergy loss (irreversibility) is

minimized. As a result, it is useful to employ the concept of

an exergetic improvement potential when analyzing different

processes of a system. Improvement potential for a

component is an avoidable portion of the exergy destruction

rate through technological (design) improvement of the

component. This improvement potential in the rate form,

denoted by IP_, is given by [80]:

. .

(1 ) xdexergyI P Eη= − (24)

The work potential of a real system can only be estimated

by defining a state corresponding to zero work potential

because the equilibrium condition is technically correct for

reference state. In exergy analysis, both mass and energy

conservation principles may be applied together for design

and analysis of different solar energy systems. The ideality,

location, type and magnitude of various losses occurred in

process can be identified and rectified accordingly.

4. Conclusion

This literature review discussed the exergy analysis of

solar powered technologies, it’s also a presentation of

different refrigeration systems powered by solar energy, that

is solar absorption and adsorption systems. The exergetic

coefficient of performance, the coefficient of performance,

the exergy loss and the exergy efficiency are the main

parameters of a refrigeration system that has been exergically

analysed. In this regard, exergy analysis is a very useful tool

which can be successfully used in the performance evaluation

of a solar refrigeration system. The solar collector which is

an important component of a solar refrigeration has also

retained our attention in this review. It is hoped that this

contribution will simulate wider interest in the exergy

analysis of solar refrigeration systems. We expected it should

be useful for any newcomer in this field of technology.

Nomenclature

COP: coefficient of performance

ECOP: exergy coefficient of performance

E: exergy (kJ/kg)

f: circulation ratio

h: enthalpy (kJ/kg)

s: entropy (kJ/kgK)

m: mass flow rate (Kg/s)

Q: heat flow rate (KW)

T: temperature (°C or K)

U: heat loss coefficient (W/m2.K)

T: temperature (°C, K)

Q: heat (kJ)

g: gravity (m2/s)

I: irreversibility (kJ/kg)

W: pump power (kW)


Nu: Nusselt number

Ra: Rayleigh number

S: radiative flux absorbed by a unit of the absorber plate

(W/m2)

Ac: aperture area (m2)

Ap: area of the absorber plate (m2)

Cp: heat capacity of the fluid (J/kg.K)

Ed: exergy loss rate (w)

I: solar irradiance (W/m2)

Subscripts

B: bottom

g: generator

e: evaporator

a: absorber

c: condenser

p: planet

h: heat

W: work

u: useful

th: thermal

ex: exergetic

el: electric

sol: solar

l: local

sun: sun

col: collector

in: inlet

total: total

out: outlet

l: loss

s: solar

Greek letters

ε: exergy

α: thermal diffusivity (m2/s)

δ: fin thickness (m)

θ: collector tilt relative to the horizontal (°)

η: efficiency

σ: Stefan-Boltzmann constant (w/m2k

4)

λ: thermal conductivity of the fins (w/m.K)

β: thermal expansion coefficient (K-1

)

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a review on exergy analysis of solar refrigeration...

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